In some transmission systems, and in particular helicopter transmission systems, there is a need to split the power (torque) developed by the engine system so that two or more load paths are provided for transfer of power to an output member. Such split power configurations reduce the tooth loading of the gear train assemblies comprising the respective load paths while concomitantly providing redundant paths for torque transmission. Should one gear assembly, i.e., load path, become inoperative, torque will be transmitted through the remaining gear assembly, thereby ensuring continued operation of the transmission system.
A schematic illustration of one embodiment of a split torque transmission system for helicopters is exemplarily illustrated in FIGS. 1, 2. Large helicopters typically utilize two or three gas turbine engines depending upon the gross weight, size, and power requirements of the helicopter, and FIGS. 1, 2 illustrate a split torque transmission system STTS for a three engine configuration. The split torque transmission system STTS transmits the power developed by the three helicopter engines to the rotor blade assembly which provides the motive power for the helicopter. The split torque transmission system STTS utilizes reduction gearing to convert engine RPM to torque for rotation of the helicopter main rotor blades.
The split torque transmission system STTS illustrated utilizes three stages of reduction gearing to reduce the RPM output of each engine (e.g., about 15,000 RPM) to an appropriate input level for the main rotor shaft (e.g., about 130 RPM). Each engine (not shown) provides an output to the transmission input via a shaft IS (which is normally coupled through a spring running clutch which has not been shown for purposes of simplification) to a gear train assembly GTA which provides torque transmission and splitting as described in the following paragraphs. A central bull gear CBG combines the split power transmitted by the gear train assembly GTA of each engine for rotation of a main rotor shaft MRS.
The first reduction stage of each gear train assembly GTA exemplarily illustrated in FIG. 1 is a bevel gear set (bevel pinion BP, bevel gear BG combination) that provides a reduction ratio of about 3.04/1 (from about 15,000 RPM to about 4934 RPM). The shaft angle of the bevel gear BG is positioned so that the centerline is parallel to the centerline of the main rotor shaft MRS. The second reduction stage is a simple spur gear set or a high contact ratio gear set (spur pinion SP, spur gear SG combination) that provides a reduction ratio of about 3.89/1 (from about 4934 RPM to about 1270 RPM). The final reduction stage is a double helical output gear set (double helical bull pinion DHP, central bull gear CBG combination) that provides a reduction ratio of about 9.77/1 (from about 1270 RPM to about 130 RPM).
Torque splitting is effected in the second stage of the gear train assembly GTA. Torque from each engine drive gear, i.e., the spur pinion SP, is split between the two second stage spur gears SG of each gear train assembly GTA. Each spur gear SG drives the corresponding integral, coaxial double helical bull pinion DHP. The central bull gear CBG combines the power from the two double helical bull pinions DHP of each gear train assembly GTA for rotation of the main rotor shaft MRS.
An inherent problem is designing such split power transmission system configurations is ensuring that torque is evenly split between the load paths, i.e., the respective spur gears SG and double helical bull pinions DHP. Any deviation in machining tolerances of the components of the gear train assemblies comprising the load paths may result in a 100% split, i.e., one load path will transmit the entire torque while the other load path will essentially free-wheel. This results in excessive wear and stress on the gear mechanisms in the load path transmitting 100% of the torque.
The above-described torque splitting configuration is one wherein load sharing is a function of the flexibility of the torsional path between the central bull gear CBG, the double helical bull pinions DHP, the second stage spur gears SG, and the second stage spur pinion SP. The configuration is a closed loop path that is extremely sensitive to the length of arc mesh. Any manufacturing errors and/or tolerance errors in the gears and/or pinions will lead to an unequal distribution of torque between the load paths.
The magnitude of these errors is proportional to the difference in length of the mesh path between the upper and lower load paths, i.e., the spur pinion SP, spur gears SG combination, and the double helical bull pinions DHP, central bull gear CBG combination, respectively, times the torsional spring rates of the paths. The greatest contributor to torsional spring rate is tooth mesh since the remaining torsional path is relatively stiff. To eliminate torque loading differences between the load paths, a torque adjusting device is typically interposed between the engine output gear, i.e., the spur pinion SP, and the central bull gear CBG to balance torque loads between the torque paths.
One prior art torque sharing device for split torque transmission systems is a quill shaft QS as exemplarily illustrated in FIG. 3. For the exemplary gear train assembly GTA illustrated in FIGS. 1, 2, the quill shaft QS is interposed between the spur gear SG and the shaft incorporating the double helical bull pinion DHP in each load path. Each spur gear SG is disposed in combination with a driven shaft DS while the corresponding double helical bull pinion DHP is integrally formed as part of a pinion shaft PS. The driven shaft DS is supported by a pair of bearings B.sub.DS and the pinion shaft PS is supported by a pair of bearings B.sub.PS.
The quill shaft QS has a helically splined end HS and a straight splined end SS which mate with splined interior portions of the pinion shaft PS and the driven shaft DS, respectively. A shim Sh permits adjustment of the gear tooth alignment between the driven shaft DS and the pinion shaft PS by means of the splined ends of the quill shaft QS. By appropriate adjustment of the quill shaft QS by means of the shim Sh, balanced torque splitting between the load paths is achieved. The quill shaft QS provides a relatively soft torsional spring rate.
While the use of quill shafts QS is effective in compensating for manufacturing variations, and thus ensuring approximately a 50-50 torque split between respective gear train assemblies, incorporation of a quill shaft QS in each gear train assembly GTA increases the overall complexity and weight of the transmission system. This, in turn, increases the costs and time required for initial transmission system assembly and subsequent maintenance. In addition, the overall reliability of the transmission system is reduced such that periodic maintenance is required on a more frequent basis.
A need exists for a load sharing device that is operative to provide substantially equal torque distribution between the load paths of a split power transmission system. Such a device should be relatively simple in construction, thereby reducing the weight and complexity of the transmission system, while enhancing the overall reliability of the transmission system.